The present invention relates generally to the recovery of subsurface coal and more particularly to a method of in situ liquefaction of a subsurface coal formation.
For more than a hundred years, crude oil and natural gas have become increasingly more important sources of energy for the civilized world. Petroleum products have been available in copious quantities and at prices so low as to capture markets long dominated by coal and other sources of energy. The rising demand for petroleum products has advanced so rapidly that the world wide petroleum industry has been hard pressed to obtain new discoveries of sufficient magnitude to avoid being overtaken by demand. Consequently, proven reserves of petroleum as measured by the number of years of future supplies, have been declining for several years. Further, new discoveries of petroleum reserves in recent years have tended to be located at great distances from population centers, thus compounding the problems of logistics and international politics.
Petroleum has a number of attractive advantages compared to other sources of energy. First, it has a high heat content, approximately six million BTU per barrel in the form of crude oil, and one thousand BTU per cubic foot in the form of natural gas. Second, it is fluid and as such may be produced continuously, transported continuously, and used as a feedstream continuously. Third, petroleum is readily separated into numerous useful products such as gasoline, lubricants, fuel oil and the like. Convenience of use, minimum residues, clean handling and the like are other favorable attributes of petroleum.
Industrialized nations have become highly dependent upon petroleum as a source of energy. The United States, for example, currently depends upon oil and natural gas for about 78% of its energy requirements, while coal supplies are about 18% hydroelectric power about 4%, and nuclear energy less than 1%.
It is unfortunate that the petroleum industry in the United States no longer can expand rapidly enough to keep ahead of the demand for its products at competitive prices. This problem is compounded by the staggering investment and facilities that consume petroleum as a source of energy, such as aircraft, automobiles, locomotives, electric generating plants, furnaces for homes and industries, and the like.
It is obvious that alternate sources of energy must be developed at a pace compatible with the short fall of petroleum. A review of the varied energy consuming devices designed for use of petroleum energy clearly shows that some devices, for example aircraft and automobiles, do not lend themselves to redesign for use of alternate sources of energy. Other devices such as home heating units are so voluminous in number and semipermanent in nature, to make conversion impractical. Other energy consuming facilities such as electric generating plants and industrial boiler plants lend themselves more readily to conversion for use of alternate fuels.
In the ideal case, alternates to petroleum (1) would have the same physical and chemical characteristics as the products replaced, (2) would be derived from a raw material in abundant supply, for example coal, and (3) would be delivered to points of use at a cost competitive with petroleum. Thus, natural gas would be replaced with synthetic natural gas, which in turn would be composed principally of methane (CH.sub.4), and crude oil would be raplaced with synthetic crude oil, which in turn would be composed of an array of hydrocarbons. Technology is currently available to produce synthetic natural gas and synthetic crude from coal but unfortunately, except for very special and limited applications, the resultant synthetics are not competitive with petroleum in terms of cost at the point of use.
For a synthetic fuel to be competitive with a natural fuel in a free market, each step in the evolution of the synthetic fuel, on an average must be competitive with each step in the evolution of the natural fuel. In the first step of production, petroleum is delivered as a fluid to the surface by (1) differential pressure from natural reservoir pressure, (2) induced pressure from the surface to the reservoir, or (3) by artificial lift, such as pumps. It is highly desirable that coal be produced in a comparable manner. To do so requires both gasification and liquefaction of coal in situ, in contrast to most current technology which gasifies and liquefies coal above ground after coal has been mined in the conventional manner.
In any attempt to create synthetic crude oil from coal, the first obstacle to be overcome is the hydrogen deficiency of coal as compared to petroleum. While crude oils vary widely in physical characteristics from oil field to oil field, all crude oils contain approximately 10% hydrogen (H.sub.2) by weight. Coals, also vary widely in physical characteristics from deposit to deposit, but the hydrogen content in each bituminous coal deposit approximates 5% by weight, with anthracite deposits containing even lower percentages of hydrogen, on a moisture and ash free basis. Liquefying coal, then, is not enough, because it must also be hydrogenated so that coal as a liquid contains hydrogen in quantities approaching that of crude oil. It will be appreciated that it is highly desirable to hydrogenate the coal in situ so that it may be pumped to the surface as a true synthetic crude oil even though, to date, coal has not even been liquefied in situ on a reliable commercial basis. It should be further realized in the in situ production of synthetic crude oil, that just as natural crude oil often has to be cleaned at the surface, i.e., dewatered and desanded, synthetic crude oil from coal will also often have to be cleaned, i.e., deashed.
While most prior art liquefaction of coal has been performed with elaborate equipment above ground, some prior art is directed toward liquefaction of coal in situ, see for example U.S. Pat. No. 2,595,979 of Pevere et al. It is well known in the art, whether attempts be made to practice it above ground or underground, that the hydrogen deficiency of coal must be corrected by the addition of hydrogen from an outside source. Processes to add hydrogen are numerous and heretofore have enjoyed the most successes, from an engineering point of view, in the controlled confines of above ground pressure vessels. Various schemes have been advanced to hydrogenate coal in situ. All have been, at best, marginal from a technical point of view and complete failures from a commercial point of view.
There are serious problems underground to be overcome before any scheme of hydrogenation will work. One of the more serious problems is that of bringing the coal in situ up to reaction temperature. The massive coal deposit is quite frigid compared to the temperature required for hydrogenation reactions at commercially acceptable reaction rates. Numerous schemes have been proposed to raise the temperature of the underground coal. One scheme proposes the placing of electric heaters underground. The number of electric heaters required and the electric power required to bring the coal deposit up to temperature are quite disproportionate to any expected benefits that might accrue. Another scheme proposes injection of superheated vapors into the coal stratum so that, upon condensation of the vapors into liquid, heat will be added through the latent heat of condensation. This results in insignificant additions of heat in hairline cracks or narrow fractures and to localized hot spots in the wider fractures. Still another scheme proposes heating the coal formation by using the heat from the exothermic reaction of hydrogenation itself. If this scheme could be initiated, it would soon be reduced to ineffectiveness due to the enormous volume of cool coal that serves as a heat sink. Still another scheme proposes very high pressures to generate heat. At best an underground coal deposit tends to be a leaky pressure vessel, and with very high pressures the leaks are accentuated, including the possibilities of blow-outs all the way to the surface.
It is virtually imperative that the total coal deposit area to be liquefied be raised to a temperature conducive to hydrogenation and liquefaction and to applicants's knowledge this has not been satisfactorily accomplished in prior art attempts at in situ hydrogenation and liquefaction.